Robsouth. I'm not sure if they are available separately. It's the whole valve assembly that has the float gauge built in. I did a quick search and didn't see a 30lb one either. You might want to look up Worthington and call and ask them. Worthington is the brand I linked in the 20lb version.
On a side note, I spoke to someone that I saw using one on a popup at a campground and he said it was surprisingly accurate and also uses one at home for his BBQ.
Ray